The invention relates to a method for determining the nitrogen oxide content in oxygen-containing exhaust gases from internal combustion engines.
When internal combustion engines are operating, exhaust gases which contain various pollutants are formed, The levels of these pollutants are dependent substantially on the composition of the fuel/air mix. Particularly in the case of operation with a lean fuel/air mix, i.e. lambda greater than 1, the level of nitrogen oxides (NOx) is high. To ensure that the exhaust emissions regulations, which are in some countries highly stringent, can be observed, it is known to use NOx storage catalytic converters. However, despite being regenerated during certain operating conditions, these NOx storage catalytic converters have only a limited storage capacity, and consequently it is not always possible to store sufficient amounts of the nitrogen oxides produced.
To combat this problem, German Patent DE 198 01 626 A1 has already proposed a method for diagnosis of a catalytic converter in the exhaust gas from internal combustion engines which has a capacity to store both oxygen and nitrogen oxides. In this method, it is provided that a first phase shift between a lowering of the oxygen concentration and a subsequent reaction of the sensor and a second phase shift between a subsequent increase in the oxygen concentration and a following reaction of the sensor are recorded. In this method, the difference in the phase shift is determined and a fault signal is stored and/or emitted if the said difference does not reach a predetermined threshold. With this method, it is not possible to influence the operation of the internal combustion engine and the level of nitrogen oxides in the exhaust gas formed during combustion.
European Patent EP 0 783 918 A1 discloses a method for lowering the nitrogen oxide content in oxygen-containing exhaust gases from internal combustion engines, in particular from diesel engines and direct-injection spark-ignition engines for motor vehicles. In this method, the nitrogen oxides are reduced by a catalytic converter with the aid of a reducing agent which is metered to the exhaust gas as a function of operating parameters. The reducing agent used is hydrogen and/or hydrocarbon, with only hydrogen being fed to the exhaust gas upstream of the catalytic converter in a first operating mode of the internal combustion engine. Both, hydrogen and hydrocarbon are fed to the exhaust gas upstream of the catalytic converter in a second operating mode and only hydrocarbon is fed to the exhaust gas upstream of the catalytic converter in a third operating mode. In this case too, it is not possible to influence the way in which the internal combustion engine operates with regard to the formation of the nitrogen oxide fraction.
The invention is therefore based on the object of providing a method for determining the nitrogen oxide content in oxygen-containing exhaust gases from internal combustion engines, by means of which it is possible to determine the nitrogen oxide emissions on the basis of the variables which actually have an influence.
This object is achieved by a method for determining the nitrogen oxide content in oxygen-containing exhaust gases from internal combustion engines.
In the development of internal combustion engines with fuel injection, it has already been attempted for some time to determine the nitrogen oxide emissions (NOx emissions) by calculation. Achieving this determination would help, for example, to precalculate the NOx emissions and with test planning and also with plausibility checks of measured values, such as indexing data and NOx values. However, the current simulation models which are used to determine the NOx emissions by calculation are altogether inadequate. Moreover, due to the extremely high demand for calculation time, these calculation models are unable to form a control algorithm for use in vehicles.
This problem is also of particular importance in connection with the use of SCR catalytic converters. The quantity of urea to be injected for a catalytic converter of this type is in a fixed ratio to the NOx emissions. From this, it can be concluded that correspondingly accurate metering of the urea is possible as a function of the accuracy with which the NOx emissions can be determined, and therefore the efficiency of the catalytic converter can be increased.
The present invention makes it possible to precisely calculate the NOx emissions, since this calculation is based on values from the variables which actually have an influence on the NOx emissions. The level of the NOx emissions from an internal combustion engine is dependent primarily on the local temperature, the oxygen concentration and the residence time of the cylinder charge in the combustion chamber. The two latter variables can be recorded relatively easily by measuring the engine speed of the air used and also the fuel quantity. On other hand, it is much more difficult to determine the gas temperature in the combustion chamber. The present invention therefore proposes using a different variable which is directly linked to the gas temperature which is of relevance to the formation of nitrogen oxides. Since the gas temperature is decisively dependent on the center of gravity of the combustion, i.e. the position where 50% of the fuel is converted in relation to the piston position TDC, it is advantageous to select the center of gravity or a similar variable, such as for example the position of the maximum energy conversion, as a reference variable for the NOx emissions. The level of the NOx emissions is calculated from this value for the center of gravity of the combustion and the values of the recorded fuel quantity and air mass, for example with the aid of neural networks.
The determination of the center of gravity of the combustion is preferably effected by measuring the combustion-chamber pressure profile. For this purpose, a pressure sensor is provided in the region of the combustion chamber. This manner of determining the center of gravity of the combustion is extremely precise. Alternatively, it is also possible to use a dedicated model for calculating the center of gravity from the start of injection to determine the center of gravity of the combustion.
If there are pressure sensors for determining the center of gravity of the combustion, there are also further advantages, in particular with regard to the monitoring of the maximum pressure for fault detection, for establishing the operating mode and the like.
In a further configuration of the invention, it is advantageous if the quantity of recirculated exhaust gas is recorded by means of a sensor and a corresponding signal is fed to the electric circuit, then this signal can be included in the calculation of the level of the NOx emissions. Furthermore, it is advantageous if the oxygen concentration in the exhaust gas is recorded and a corresponding signal is fed to the electric circuit and if this signal is included in the calculation of the level of the NOx emissions. To monitor all the cylinders and to carry out a comparison of the corresponding pressure profiles for the purpose of fault detection, it is advantageous for a pressure sensor to be arranged in each cylinder, so that the pressure profile in the combustion chamber is recorded in each cylinder, and a separate calculation of the NOx emissions takes place for each cylinder.
Furthermore, in the case of fast-running internal combustion engines, it is expedient for the rotational speed of the internal combustion engine to be recorded and for a corresponding signal to be fed to the electric circuit, and for this signal to be included in the calculation of the level of the NOx emissions. Moreover, it is expedient to provide an NOx sensor which records the NOx content in the exhaust-gas stream, the resulting measured value being compared with the level of the calculated NOx emissions.